1. Field of the Invention
The present invention relates to a liquid crystal display unit, and more particularly to a liquid crystal display unit using a photoconductive liquid crystal light valve.
2. Description of Background Information
FIG. 1 shows the principle of a projection type liquid crystal display unit, for example, using a photoconductive liquid crystal light valve. In this Figure, an image is written on the photoconductive liquid crystal light valve by the writing light emitted from the writing optical system 2. On the other hand, the light emitted from a light source 3 enters the polarization beam splitter 4. Of the incident light, S polarized light component is deflected at a right angle by a polarization beam splitter 4, and it enters the photoconductive liquid crystal light valve 1 as the reading light.
If an image is given on the liquid crystal layer of the photoconductive liquid crystal light value 1, the light reflected at the photoconductive liquid crystal light valve 1 contains P polarized component locally according to the density of the image on the liquid crystal layer. When only P polarized component of this reflected light passes through the polarization beam splitter 4, the image is projected on a screen 6 through a projection lens system 5 by this P polarized component.
FIG. 2 shows the structure of the photoconductive liquid crystal light valve 1 used on this projection type liquid crystal display unit. In this Figure, a spacer 12 is placed around the liquid crystal layer 11, and liquid crystal orientation layers 13 and 14 are provided on both sides of the liquid crystal layer 11. The photoconductive layer 15 consists, for example, of amorphous silicon (hereinafter abbreviated as "a-Si"), and it is laminated on the liquid crystal orientation layer 13 through a light reflection layer 16 and a light cutoff layer 17. On the outer lateral side of the liquid crystal orientation layer 14, a transparent electrode 18 made of indium tin oxide (ITO) is disposed, while a transparent electrode 19 made of tin dioxide (SnO.sub.2) is arranged on the outer lateral side of the photoconductive layer 15. These elements are sealed by a pair of glass substrates 20 and 21.
In a photoconductive type liquid crystal light valve 1 of such a structure, an AC voltage is applied between the transparent electrodes 18 and 19. If the internal impedance of the photoconductive layer 15 is in a dark condition, that is, when no writing light is irradiated from the left side (write side) of the Figure, is set to a value sufficiently higher than that of the liquid crystal layer 11, the AC voltage is applied mainly on the photoconductive layer 15. When the writing light is irradiated and an image is given on the photoconductive layer 15 by this writing light, the internal impedance of the photoconductive layer 15 is locally decreased according to the density of the image. Thus, on the liquid crystal layer 11 adjacent to this decreased portion, the AC voltage applied between the transparent electrodes 18 and 19 is modulated spatially according to the density of the image, so that the image is written on it.
The ratio between voltages applied on the liquid crystal layer 11 when the writing light is irradiated and when it is not irradiated is called a light switching ratio, and the light switching ratio is a parameter to indicate the operating status of the photoconductive type liquid crystal light valve.
In the writing optical system 2 (FIG. 1) of this photoconductive type liquid crystal light valve 1, a monochromatic light is used as the writing light. On the other hand, to increase the light switching ratio, the thickness of the photoconductive layer 15 consisting of a-Si and the like must be several .mu.m. However, both light absorption and photoconductive sensitivities of such a thick photoconductive layer 15 show a wavelength dependency. Namely, as it is evident from FIG. 3, no absorption occurs for the longer wavelength of 700 nm or more, and sensitivity is rapidly decreased. On the other hand, absorption is limited to the surface layer for the shorter wavelength. Thus, photoconductive effect does not occur on the entire thick photoconductive layer 15, and this also leads to the decrease of sensitivity. Therefore, to give an optimal sensitivity, there must be restrictions to the thickness of the photoconductive layer 15 and to the wavelength of the light.